Known electric motor systems typically include a motor and a control unit for controlling power to the motor. Known types of electric motor include the induction motor, brushless permanent magnet motor, switched reluctance motor and synchronous slip ring machine. Three phase electric motors are the most common kind of electric motor available.
FIG. 1 shows a schematic representation of a typical three phase motor. In this example, the motor includes three coil sets. Each coil set produces a magnetic field associated with one of the three phases of the motor. In a more general example, N coil sets can be used to produce an N-phase electric motor. Each coil set can include one or more sub-sets of coils which are positioned around a periphery of the motor. In the present example, each coil set includes four such sub-sets—the coil sub-sets of each coil set are labelled 14, 16 and 18, respectively in FIG. 1. As shown in FIG. 1, the coil sub-sets 14, 16, 18 are evenly distributed around the motor 10 to co-operate in producing a rotating magnetic field within which a central rotor 12, which typically incorporates one or more permanent magnets, can rotate as shown by the arrow labelled C. The coil sub-sets of each coil set are connected together in series as shown by the connections 24, 26 and 28 in FIG. 1. This allows the currents in the coils of each coil set to be balanced for producing a substantially common phase. The wires of each coil set are terminated as shown at 34, 36 and 38 in FIG. 1. Typically, one end of the wire for each coil set is connected to a common reference terminal, while the other wire is connected to a switching system for controlling the current within all of the coils of that coil set. Typically then, current control for each coil set involves controlling a common current passing through a large number of coils.
As shown in FIG. 2, each coil sub-set can include one or more coils. In particular, FIG. 2 shows the coils 24A, 24B in one of the coil sub-sets 14. In this example, there are two coils per coil sub-set. The two coils are wound in the opposite directions, and are interconnected so that the current flowing in each coil is substantially the same. As the poles of the rotor 12 sweep across the coils 24A, 24B, switching of the current in the coils 24A, 24B can produce the appropriate magnetic field for attracting and repelling the rotor for continued rotation thereof. The magnetic field produced by the two oppositely wound coils 24A, 24B is referred to as belonging to the same phase of this three phase motor. Every third coil sub-set arranged around the periphery of the motor 10 produces a magnetic field having a common phase. The coils and the interconnections may typically comprise a single piece of wire (e.g. copper wire) running around the periphery of the motor and wound into coils at the appropriate locations.
For a three phase electric motor, the switching system is almost invariably a three phase bridge circuit including a number of switches.
Typical power electronic switches including the Metal Oxide Silicon Field Effect Transistor (MOSFET) and the Insulated Gate Bipolar Transistor (IGBT) exhibit two principal losses: switching losses and conduction losses.
While switching losses decrease with switching speed, a faster switching speed also leads to increased electromagnetic interference (EMI) noise. This problematic trade off between switching speed and EMI noise is compounded at higher power ratings (e.g. for a larger motor), since larger switches are required. The inductance associated with a power switch and its connection system increases with the physical size of the switch. This inductance impacts the switching speed of the power device and the switching speed of a power device is typically therefore limited by its physical size. Accordingly, for high power ratings larger switches must be used, but larger switches involve slower switching speeds and therefore larger switching losses. Moreover, the cost of a power device increases roughly with the square of the size of the device. Conduction losses also increase with increased power.
Including switching losses and conduction losses, the total losses are approximately proportional to the square of the power. This imposes serious thermal management problems for the motor since, for example, a doubling of the power leads to a four fold increase in thermal losses. Extracting this heat without elevating the temperature of the device above its safe operating level becomes the limiting factor in what power the device can handle. Indeed, today larger power devices having intrinsic current handling capabilities of, for example, 500 A are restricted to 200 A due to thermal constraints.
Consider a conventional three phase motor with a given power rating. If a larger power rating is desired, this can be achieved by producing a motor with a larger diameter. For a larger motor diameter, the peripheral speed of the rotor increases for a given angular velocity. For a given supply voltage this requires that the motor coils to have a reduced number of turns. This is because the induced voltage is a function of the peripheral speed of the rotor and the number of turns in the coils. The induced voltage must always be at or below the supply voltage.
However, the reduced number of turns in the coils leads to a reduced inductance for the motor, since the inductance of the motor is proportional to the square of the number of turns.
Almost all electronic control units for electric motors today operate by some form of pulse width modulation (PWM) voltage control. PWM control works by using the motor inductance to average out an applied pulse voltage to drive the required current into the motor coils. Using PWM control an applied voltage is switched across the motor windings for a minimum period dictated by the power device switching characteristic. During this on period, the current rises in the motor winding at a rate dictated by its inductance and the applied voltage. The PWM control is then required to switch off before the current has changed too much so that precise control of the current is achieved.
As discussed above, the use of larger power devices leads to a slower switching speed, while a larger motor also has a lower inductance. For higher power motors, these two factors inhibit the effectiveness of PWM as a control system because the current in the motor coils rises more rapidly (due to the low inductance of the motor due to the fewer number of turns in the coils) but the PWM control is more coarse (due to the slow switching speed achievable using high power switching devices).
A known solution to this problem is to introduce additional inductance in the motor in the form of current limiting chokes in series with the motor windings. This added inductance increases the rise time of the current in the motor coils. However, the chokes are typically as large or larger than the motor itself and as they carry the full current they dissipate a large additional heat loss as well as being a substantial extra volume, weight and cost.
Other problems with known motors relate to their manufacture. As described above in relation to FIG. 1, motor construction typically involves using a single length of wire to produce the windings for each phase of the motor. The wire runs around a periphery of the motor and coils are wound at the appropriate locations for producing a phase of the magnetic field of the motor. Winding the coils of the motor, as well as terminating the connections between each coil sub-set interspersed around the motor periphery is a labour intensive task. The thick wire (e.g. copper wire) typically used in motor windings is difficult to manipulate and in many motor designs, access to the innards of the motor for installing the coils and their interconnections is limited. Known coil mounting systems are also bulky and have limited heat dispersing capabilities.
Vehicle traction control can be used for minimizing the risk of skids which can occur while the vehicle is moving. A vehicle relying on wheel traction to provide a resultant locomotive force suffers from the phenomenon of wheel skid. Steering skids can also occur. In a steering skid, the motion of the vehicle is out of alignment with that of the front wheels (commonly known as under-steer) or the rear wheels (over-steer).
In general, the onset of a skid is not a sudden event, but starts with a degree of wheel slip, which then builds up to a full wheel skid. The amount of force needed to produce a wheel slip or skid can be calculated by the weight on the wheel multiplied by a coefficient of friction between the tyre and the road surface. If this force is exceeded, then a wheel slip or skid will occur. At forces just below the force at which wheel slip or skid can occur, maximum drive performance is being obtained while the wheel is still in grip. Traction control systems generally aim to allow operation in this region, whereby maximum force can be applied to the wheels with allowing wheel slip or skid to occur.
In known systems, torque is applied to the wheels of a vehicle from a central internal combustion engine through a drive shaft and differential gears. Traction control is normally applied through modulating the brake discs pressure (for braking) or by modulating a slip clutch mechanism by each wheel (for acceleration). These traction control systems require expensive mechanical parts and do not always provide the best performance. For example, ABS brakes tend to shudder violently as they are operated on a crude on/off basis. Slip clutches have an effect on left/right torque balancing from the engine.